Update: SIS Plans Quad-Channel Rambus Chipset

SAN JOSE – Following on the heels of its dual-channel Rambus-enabled chipset, Silicon Integrated Systems Inc. officials confirmed that SIS is developing a four-channel model set to sample later this year.

The SIS R659, the sequel to the SIS dual-channel R658 chipset, is set to sample in the third quarter, according to a SIS spokeswoman. A formal announcement is due “very soon”, the SIS spokeswoman said, but later amended her statement to say that the R659 may be announced after the Chinese New Year next week.

Wednesday night, SIS included a mention of the R659 in a separate release desctibing its 1-Gbyte/s MULTIOL technology connecting its north and south bridges.

“Apart from the Platform Conference, SiS this week is launching its next-generation Rambus chipset, the SiSR659, designed to support four-channel RDRAM PC1200,” the company said in a statement. “This latest Rambus solution from SiS pairs the SiSR659 with the SiS964 in the south bridge, which integrates USB 2.0 with up to 8 ports and Serial ATA features. Sample delivery of the SiSR659 is scheduled for the third quarter of 2003.”

Four-channel Rambus memory would swing the balance of the platform away from the CPU and back toward the front-side bus, if not balance it altogether. If Intel uses a quad-pumped 200-MHz front-side bus to generate its 800-MHz effective clock rate, the sum total bandwidth available would be 6.8 Gbytes/s, versus the 8.4 Gbytes/s produced by the four-channel PC1066 Rambus DRAM.

“As CPUs get faster, the imbalance between CPU performance and memory bandwidth just gets worse,” noted ExtremeTech analyst Loyd Case. “A quad-channel memory setup would give the P4 more bandwidth, which it can generally use. Note that Intel will be moving to an800MHz frontside bus in a couple of months, according to published roadmaps, which means that SiS will be able to take advantage of that quite nicely with their new chipset.”

The chipset’s timing could also be complicated by a management shift within SIS. According to a SIS spokeswoman, foundry United Microelectronics Corp. now holds a 30 percent stake in SIS. United Microelectronics Corp. vice chairman John Hsuen has taken over the helm of chipset maker Silicon Integrated Systems as acting president, following Samuel Liu’s removal from his positions as chairman, chief executive officer and president.

Although the R659 has yet to be formally disclosed, the discussion of the chipset’s capabilities popped up repeatedly during the memory track here at the Platform Conference, which featured multiple presentations by executives at Samsung, one of the top Rambus proponents.

According to Jim Elliott, senior product marketing manager of DRAM for Samsung, four-channel PC1066 RDRAM would yield a theoretical memory bandwidth of 8.4 Gbytes per second, substantially more than the 6.4 Gbytes/sec theoretical memory bandwidth produced by dual-channel DDR-400, a niche product which is just beginning to roll out. Future dual-channel 1.3-GHz Rambus memory could yield 10.7 Gbytes/s of theoretical memory bandwidth.

“We really see a 4-channel chipset as a compelling solution for the future to embrace the PC,” Elliott said.

Four-channel Rambus memory would actually have a slightly larger advantage over DDR-333 memory when used with an 800-MHz front-side bus, Elliott added. Because the multipliers used in DDR-333 memory don’t match up with those used to create an 800-MHz bus, the DDR-333 memory must actually be downclocked to 320 MHz, lowering the bandwidth from 5.3 to 5.1 Gbytes per second., Elliott said.

Several hurdles have yet to be overcome. Samsung, for example, did not overtly indicate when four-channel capable modules could be manufactured. Even if they are produced, it is likely that SIS will have to perform the module validation itself, if Intel declines to do so. When the first PC1066 modules were introduced by Kingston early last year, Intel had not yet validated them.

In addition, there is the issue of cost. Rambus modules have traditionally commanded a sizeable price premium, although the price has begun to drop in parallel with DDR DRAM modules. So far, Samsung is also the only supplier to promote its plans for four-channel Rambus memory.

The R659 is a “noisemaker,” said Bert McComas, an analyst with InQuest Market Research, the primary sponsor of the show. SIS may be promoting the chipset to secure a license to Intel’s 800-MHz front-side bus, he said.

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